1. Field of the Invention
The present invention relates to a radio communication apparatus and a radio communication method in which transmission and reception are simultaneously performed using the same carrier frequency.
2. Related Background Art
While different time-based allocation (time slots) for transmission and reception are employed to prevent jamming in TDD systems used for conventional radio communication in which the same RF (Radio Frequency) carrier frequency is used for transmission and reception, US Laid-open Gazette Patent Application No. 2004/0142700 discloses a method in which, with an object of improving frequency utilization efficiency, the transmission and reception are simultaneously performed in the same time slot.
In this method, interference occurs due to imperfections in a circulator which results in signals from a transmission RF signal processor leaking into a received RF signal processor. Further interference is caused by the effects of reflection and diffraction and so on of the propagation path that result in the transmitted signal transmitted from an antenna being received by the same antenna. These interferences are referred to as “coupling loop interference”. Employment of a signal consisting of the coupling loop interference signal described above overlaid on a desired signal transmitted from one radio communication apparatus to another radio communication apparatus requires that the desired signal be detected by the receiving radio communication apparatus, however if the coupling loop interference signal power is large the desired signal cannot be accurately detected. Notably, the effect of coupling loop interference attributable to imperfections of the circulator is comparatively much worse than the effect of coupling loop interference caused by a signal being received again once it has been transmitted from the antenna.
With this in mind, US Laid-open Gazette Patent Application No. 2004/0142700 discloses a method of interference removal in a baseband digital domain employing a radio communication apparatus with a coupling loop interference signal removal function.
In addition, another radio communication apparatus able to be used to remove these interferences is a radio communication apparatus with a coupling loop interference signal removal function described as a booster. The cited references “Suzuki, Ebine, “Configuration and characteristics of an open space booster device for radio paging systems”, 1996 IEIEC Annual Conference, B-428, pp. 428, March 1996” (hereinafter referred to as “cited reference 1”) and “H. Suzuki, K. Itoh, Y. Ebine, M. Sato, “A booster configuration with adaptive reduction of transmitter-receiver antenna coupling for pager systems”, Proc. of 50th IEEE Vehicular Technology Conference, VTC 1999-Fall, vol. 3, pp. 1516-1520, September 1999” (hereinafter referred to as “cited reference 2”) describe boosters for radio paging systems notable in that interference removal in an RF band which constitutes the analog domain and interference removal in the baseband band which constitutes the digital domain are simultaneously performed. In addition, the cited reference “Imamura, Hamasumi, Shibuya, Sasaki, “A fundamental study of loop coupling cancellers for broadcast-wave relay in digital terrestrial broadcasting SFN”, The Journal of the Institute of Image Information and Television Engineers, vol. 54, No. 11, pp. 1568-1575, 2000” (hereinafter referred to as “cited reference 3”) describes a booster for digital terrestrial broadcasting that performs interference removal at the IF band or baseband band. Furthermore, the cited reference “Maeyama, Inoue, Kamimura, Yamato, “Development of a CDMA repeater comprising an interference wave reducing function”, 2002 IEIEC Annual Conference, B-5-62, pp. 512, March 2002” (hereinafter referred to as “cited reference 4”), describes a CDMA signal booster that performs interference cancellation only at the RF band.
However, the following problems are inherent to the radio communication apparatus of the conventional art described above. That is to say, in the conventional radio communication apparatus described above an analog domain signal consisting of a coupling loop interference signal overlaid on a desired signal intended for reception is processed by a received RF signal processor of a radio communication apparatus with a coupling loop interference signal removal function. Because of the limit value on the magnitude of the input power for correct operation of each of the received RF signal processors of a signal amplifier, filterer and frequency converter (down converter), when the coupling loop interference signal power is significantly larger than the desired signal power, the input power exceeds the limit value and a saturation of the signal occurs. As a result, a marked deformation of the signal wave shape occurs in the signal amplifier, filter and frequency converter in the received RF signal processors. At this time, the greater the magnitude of the input power with respect to the limit value, the greater the signal saturation effect and the greater the deformation of the signal wave shape.
In addition, while as disclosed in cited reference 1 and cited reference 2 noted above, the signal saturation effect at the received RF signal processors can be reduced by removing the interference in the analog domain prior to the signal being received by the received RF signal processors, in this case as well, because of the limit value of the magnitude of the input power for correct operation of the interference remover in the analog domain, signal saturation occurs when a signal in which the limit value is exceeded is input into the interference remover of the analog domain. When deformation of the signal wave shape occurs due to the signal saturation effect in this way, the interference removal characteristics produced by the interference removal performed at the analog domain which constitutes the RF band and the digital domain which constitutes the baseband band are insufficient and, accordingly, a marked deterioration in the detected characteristics of the desired signal occurs.